Thixotropic compositions are useful as coatings in many applications including the automotive industry where they are used as undercoating materials and interior cavity protective films.
A composition having thixotropic properties has a reduced viscosity under high shear conditions and a higher viscosity under low shear conditions. These properties are particularly useful in applications where it is desired to apply a normally viscous composition to surfaces using spraying equipment that, after spraying, results in adherence of the compositions to the surfaces. In the particular application of an undercoating material, in order to be effective as an undercoating material, the compositions should have spray properties enabling uniform spraying and atomization properties. In addition, other physical properties should provide appropriate properties of adhesion, cure time, sag (resistance to flow on vertical surfaces), heat-stability (sag at elevated temperature), film continuity as well as anti-corrosion and sound deadening properties.
Many coating compositions have been developed in the past and the market is well supplied with different products, many of which have unique properties and chemistries. As a result, there are a large class of compositions that provide some or many of the above properties.
From an economic or commercial perspective, there continues to be a need for thixotropic compositions that provide improvements in the above properties and that are economic to manufacture. That is, with the cost of raw materials and manufacturing processes affecting the cost to the consumer, there continues to be a need for protective coating compositions that remain competitive within the marketplace. In particular, there is a need for thixotropic compositions that are produced by a simplified and reliable process using readily available, economical and non-hazardous raw materials with simplified equipment and production times.
A review of the prior art indicates that in the past, many thixotropic compositions have been prepared by methodologies that result in various forms of calcium carbonate/calcium sulfonate mixtures having properties that impart corrosion resistance to metal surfaces. However, in many of these past processes, the use of other ingredients, such as promoters, have been required to achieve various chemical reactions, impart specific physical properties and/or to enable the creation of a stable colloidal suspension. Generally, surfactant materials (ie. oil soluble long-chain carboxylate salts and/or sulfonate salts) are required to make non-polar oil-like materials more compatible with polar inorganic salts (Ca(OH)2 and CaCO3) to enable the creation of a colloidal suspension of oils and the salt complexes.
Some of these past processes substitute all or part of the calcium sulfonate with calcium salts of various types of carboxylic acids. For example, U.S. Pat. No. 4,597,880 describes thixotropic compositions including short-chain water-soluble carboxylic acids that function as promoters to achieve needed chemical reactions and/or physical processes to enable calcium carbonate to be distributed as a colloidal suspension in oil-like carrier materials in a form which is sufficiently finely divided so as not to settle out.
Importantly, the advantages of eliminating promoter materials include:                a. The cost of using a material which has no functionality in the final product is eliminated;        b. The promoter materials are generally low flash organic materials (eg. alcohols) which require plant equipment for containment, ventilation etc. for safety and environmental reasons; and,        c. There is evidence that these promoters interfere with the subsequent stage of producing the thixotropic materials, which is transforming the colloidal suspension into a gelled material. As a result, several processes may be required to strip the promoter materials out before proceeding to the next stage.        
Moreover, past thixotropic compositions all disclose the use of sulfonic acids having a minimum aliphatic carbon chain length of 12 carbon atoms that are less reactive and are more expensive.
Further still, past processes have been made complex through manufacturing processes requiring the formation of CaCO3 “in situ” by reaction of excess Ca(OH)2 with CO2 gas in order to obtain the necessary finely divided, and completely dispersed calcium carbonate particles that enable a colloidal dispersion. Thus, there has been a need for a process utilizing the addition of solid CaCO3 that provides the desired physical/chemical results as well as the economic advantages of utilizing a single-step mixing process as opposed to a multiple-step chemical process.
As an example, Canadian Patent 2,057,196 describes longer chain (C8-C24) carboxylic acids in combination with oil soluble sulfonic acids neutralized to calcium salts with excess calcium hydroxide. In this patent, a calcium carbonate complex is produced by reaction of excess calcium oxide (or calcium hydroxide) with carbon dioxide gas introduced to the reaction mixture. This process has been described as necessary to obtain the calcium carbonate in the appropriately finely divided crystalline form. Furthermore, in this process, an alcohol “reaction promoter” is also utilized to form an initial “oil soluble dispersing agent”.
Other prior art patents include U.S. Pat. No. 3,816,310 which discloses a method for preparing a rust inhibiting composition that contains oil soluble metal salts of sulfonic acids, carboxylic acids, and phosphorous sulfide treated olefins; U.S. Pat. No. 4,597,880 which discloses a one-step process for preparing a thixotropic calcium sulfonate complex containing calcium carbonate with calcium sulfonate being a dispersing agent; U.S. Pat. No. 5,407,471 which discloses a process for inhibiting the corrosion of metal by applying a coating containing an organic acid and at least one metal containing corrosion inhibitor; U.S. Pat. No. 4,161,566 which discloses the formation of an aqueous dispersion composition of irreversibly formed films by reacting a carboxylic acid with an overbased salt; U.S. Pat. No. 4,629,753 which discloses a water dispersed rust inhibiting composition comprising a film forming organic polymer and a non-Newtonian dispersion system comprising colloidal particles, a dispersing medium and a hydrophobic organic compound; U.S. Pat. No. 4,479,981 which discloses a thixotropic water reducible corrosion resistant coating containing carboxylic acid, an overbased sulfonate and an alcoholic coupling solvent such as propyl glycol ether and water.